A known radio frequency passive acoustic transponder system provides a radio-frequency surface acoustic wave on a piezoelectric substrate which interacts with elements on the substrate to produce an individualized complex waveform response to an interrogation signal. The code space for these devices may be, for example, 2.sup.16 codes, or more, allowing a large number of transponders to be produced without code reuse. These devices consist of a piezoelectric substrate on which a metallized conductor pattern is formed, for example by a typical microphotolithography process, with a minimum feature size of, for example, one micron, and appropriate antennas and mechanical enclosures. The acoustic wave mode is often a surface acoustic wave (e.g., a Rayleigh wave), although acoustic wave devices operating with different wave types are known.
The known transponder devices thus include a surface acoustic wave device, in which an identification code is presented as a characteristic time-domain delay pattern in signal retransmitted from the transponder. Typical systems generally require that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is easily distinguished from the emitted signal during the entire duration of the retransmitted signal return, representing a plurality of internal states of the transponder, allowing analysis of the various delay components within the device.
In such a device, received RF energy is transduced onto a piezoelectric substrate as an acoustic wave with a first interdigital electrode system, from whence it travels through the substrate, interacting with reflector, delay or resonant/frequency selective elements in the path of the acoustic wave, resulting in specific known electro-acoustic interactions. A portion of the acoustic wave energy is ultimately received an interdigital electrode system and retransmitted. The retransmitted signal thus represents a complex delay and attenuation pattern function of the emitted signal, and a receiver is provided which analyzes the delay and perturbation pattern to characterize the system which produced it; thus identifying the device.
These devices do not require a semiconductor memory nor external electrical energy storage system, e.g., battery or capacitor, to operate. The propagation velocity of an acoustic wave in such a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, the time for transmission between the radio frequency interrogation system and the transponder is typically short as compared to the acoustic delay intrinsic to the device, so that an allowable rate of the interrogation frequency change is based on the delay characteristics within the transponder. The interrogation frequency is controlled to change by a sufficient amount so that the shortest possible delay path of a return signal may be distinguished from the simultaneous interrogation frequency, and so that all of the relevant delays are unambiguously received for analysis. Further, the interrogation frequency should not return to the same frequency before a maximum delay period, thus preventing ambiguity or aliasing. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217.
In its simplest form, the acoustic transponder systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of an electrical signal corresponding to the RF interrogation wave, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. This acoustic wave may be modified along its path, such as by reflection, attenuation, variable delay (phase shift), and interaction with other transducers or resonators.
When an acoustic wave pulse is reconverted into an electrical signal, it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. The signal may be reflected back along its incident path, and thus a single antenna and transducer may be provided, for both receiving and emitting Radio Frequency energy. This energy is received at a receiver and decoder, typically at or near the same location as the interrogating transmitter, and the information contained in this response to an interrogation signal is decoded. Designs are known, with unitary and separate receiving and transmitting antennas, which may be at the same frequency or harmonically related, and having the same or different polarization.
In systems of this general type, the information code associated with and which identifies the passive transponder is built into the transponder at the time that the metallization pattern is finally defined on the substrate of piezoelectric material. This metallization also typically defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g., reflectors. Thus, the information code in this case is non-volatile and permanent. The information is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as a specific complex delay pattern and attenuation characteristics. In the case of a transponder tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is N.times.2.sup.M where N is the number of acoustic waves launched by the transducers (path multiplicity) and M is the number of reflective element positions for each transducer (codespace complexity). Thus, with four launch transducers each emitting two acoustic waves (forward and backward) (N=8), and a potential set of eight (M=8) variable reflective elements in each acoustic path, the number of differently coded transducers is 2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, efficiency is lost with increasing complexity, and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each. Therefore, the transponder design is a tradeoff between device codespace complexity and efficiency.
Typically, the sets of reflective elements in each path form a group, having a composite transfer function, while each group, representing different acoustic paths, has a different characteristic timing, allowing the various group responses to be distinguished.
The transponder tag thus typically includes a multiplicity of "signal conditioning elements", i.e., delay elements, reflectors, and/or amplitude modulators, which are coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the acoustic wave interacting with it. Even where the signal is split into multiple portions, it is advantageous to reradiate the signal through a single antenna. Therefore, a single "signal combining element" coupled to the all of the acoustic waves, which have interacted with the signal conditioning elements, is provided for combining the intermediate signals to produce the radiated transponder signal. The radiated signal is thus a complex composite of all of the signal modifications, which may occur within the transponder, of the interrogation wave.
In known passive acoustic transponder systems, the transponder remains static over time, so that the encoded information is retrieved by a single interrogation cycle, representing the state of the tag, or more typically, obtained as an inherent temporal signature of an emitted signal due to internal time delays. In order to determine a transfer function of a passive transponder device, the interrogation cycle may include measurements of excitation of the transponder at a number of different frequencies. This technique allows a frequency domain analysis, rather than a time domain analysis of an impulse response of the transponder. Essentially, the composite response of M signal conditioning elements within the transponder tag are evaluated at at least M different frequencies, allowing characterization of the group of elements. Displaced in time from each other, N groups of elements may be analyzed during the same interrogation sequence.
Typically, the interrogator transmits a first signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, referred to herein as the nominal 915 MHz band, a frequency band that is commonly available for such use. The response of the tag to excitation varies with frequency, due to the fixed time delays and attenuation. In some known systems, the excitation frequency changes over time, so that the retransmitted response, due to the acoustic propagation delay of the tag, is at a different frequency than the simultaneously emitted signal, thus providing a retransmitted signal removed slightly from the emitted signal, so that when cross-modulated, the resulting signal is near baseband, but not DC.
Preferably, the passive acoustic wave transponder tag includes at least one element having predetermined characteristics, which assist in synchronizing the receiver and allows for temperature compensation of the system. As the temperature changes, the piezoelectric substrate may expand and contract, altering the characteristic delays and other parameters of the tag. Variations in the transponder response due to changes in temperature thus result, in part, from the thermal expansion of the substrate material. Although propagation distances are small, an increase in temperature of only 20.degree. C. can produce an increase in propagation time by the period of one entire cycle at a transponder frequency of about 915 MHz; correspondingly, a change of about 1.degree. C. results in a relative phase change of about 18.degree.. The potential range of variation in an uncontrolled environment therefore requires an internal temperature reference/compensation mechanism.
This known sequential frequency excitation (chirp) interrogation surface acoustic wave transponder system provides a number of advantages, including high signal-to-noise performance, and the fact that the output of the signal mixer at the interrogator receiver--namely, the signal which contains the difference frequencies of the interrogating chirp signal and the transponder reply signal--may be transmitted over inexpensive, shielded, twisted-pair wires because these frequencies are, for example, typically in the audio range. Furthermore, since the audio signal is not greatly attenuated or dispersed when transmitted over long distances, the signal processor may be located at a position quite remote from the signal mixer, or provided as a central processing site for multiple interrogator antennae.
Passive transponder encoding schemes include selective modification of interrogation signal transfer function H(s) and delay functions f(z). These functions therefore typically generate a return signal in the same band as the interrogation signal. Since the return signal is typically mixed with the interrogation signal, the difference between the two will generally define the information signal for analysis, along with possible interference and noise. By controlling the rate of change of the interrogation signal frequency with respect to a maximum round trip propagation delay, including internal delay, as well as possible Doppler shift, the maximum bandwidth of the demodulated signal may be controlled. Thus, the known systems employ a chirp interrogation waveform which allows a relatively simple processing of limited bandwidth transponder signals.
Known surface acoustic wave passive interrogator label systems, as described, for example, in U.S. Pat. Nos. 4,734,698; 4,737,790; 4,703,327; and 4,951,057, include an interrogator comprising a voltage controlled oscillator which produces a first signal at a radio frequency determined by a control voltage supplied by a control unit. This signal is amplified by a power amplifier and applied to an antenna for transmission to a transponder. The voltage controlled oscillator may be replaced with other oscillator types.
For example, as shown in FIG. 2, the signal S1 is received at the antenna 24 of the transponder 20 and split into a number of subsignals I.sub.N by combiner 42. The subsignals are each subject to a different signal modification element A.sub.N (f), T.sub.N (f) 40, and returned to the combiner 42. Each signal modification element 40 converts a portion of the first (interrogation) signal S1 into a second (reply) signal S2, encoded with an information pattern. The signal conditioning elements 40 are selectively provided to impart a different response code for different transponders, and which may involve separate intermediate signals I.sub.0, I.sub.1 . . . I.sub.N within the transponder. Each signal conditioning element 40 comprises a known delay T.sub.i and a known amplitude modification A.sub.i (either attenuation or amplification). The respective delay T.sub.i and amplitude modification A.sub.i may be functions of the frequency of the received signal S1, constant independent of frequency, or have differing dependency on frequency. The order of the delay and amplitude modification elements may be reversed; that is, the amplitude modification elements A.sub.i may precede the delay elements T.sub.i. Amplitude modification A.sub.i can also occur within the path T.sub.i. The modified signals are combined in combining element 42 which combines these intermediate signals (e.g., by addition, multiplication or the like) to form the reply signal S2 and the combined signal emitted by the antenna 18.
The information pattern is thus encoded as a series of elements having characteristic delay periods T.sub.0 and .DELTA.T.sub.1, .DELTA.T.sub.2, . . . .DELTA.T.sub.N. Two common types of encoding systems exist. In a first, the delay periods correspond to physical delays in the propagation of the acoustic signal. After passing each successive delay, a portion of the signal I.sub.0, I.sub.1, I.sub.2 . . . I.sub.N is tapped off and supplied to a summing element. The resulting signal S2, which is the sum of the intermediate signals I.sub.0 . . . I.sub.N, is fed back to a transponder tag antenna, which may be the same or different than the antenna which received the interrogation signal, for transmission to the interrogator/receiver antenna. In a second system, the delay periods correspond to the positions of reflective elements, which reflect portions of the acoustic wave back to the launch transducer, where they are converted back to an electrical signal and emitted by the transponder tag antenna. The signal is passed either to the same antenna or to a different antenna for transmission back to the interrogator/receiver apparatus. The second signal carries encoded information which, at a minimum, serves to identify the particular transponder.
The modified transponder (second) signal is picked up by antenna 56, as shown in FIG. 7. Both this second signal and the first signal (or respective signals derived from these two signals) are applied to a mixer 68 (four quadrant multiplier) to produce a third signal S3 containing frequencies which include both the sums and the differences of the frequencies contained in the signals S1 and S2. The signal S3 is passed to a signal processor 102 which determines the amplitude a.sub.i and the respective phase .phi..sub.i of each frequency component f.sub.i among a set of frequency components (f.sub.0, f.sub.1, f.sub.2 . . . ) in the signal S3. Each phase .phi..sub.i is determined with respect to the phase .phi..sub.0 =0 of the lowest frequency component f.sub.0. The signal S2 may be intermittently supplied to the mixer by means of a switch (not shown), and indeed the signal processor may be time-division multiplexed to handle a plurality of S2 signals from different antennas 56.
The information determined by the signal processor 102 is passed to a computer system comprising, among other elements, a random access memory (RAM) 104 and a microprocessor 106. This computer system analyzes the frequency, amplitude and phase information of the demodulated signal and makes decisions based upon this information. For example, the computer system may determine the identification number of the interrogated transponder 20. This I.D. number and/or other decoded information is made available at an output 107 to host computer 108.
In one known interrogation system embodiment, the voltage controlled oscillator 72 is controlled to produce a sinusoidal RF signal with a frequency that is swept in 128 equal discrete steps from 905 MHz to 925 MHz. Each frequency step is maintained for a period of 125 microseconds so that the entire frequency sweep is carried out in 16 milliseconds. Thereafter, the frequency is dropped back to 905 MHz in a relaxation period of 0.67 milliseconds. The stepwise frequency sweep 46 shown in FIG. 3B thus approximates the linear sweep 44 shown in FIG. 3A.
Assuming that the stepwise frequency sweep 44 approximates an average, linear frequency sweep or "chirp" 47, FIG. 3B illustrates how the transponder 20, with its known, discrete time delays T.sub.0, T.sub.1 . . . T.sub.N, produces the second (reply) signal S2 with distinct differences in frequency from the first (interrogation) signal S1. Assuming a round-trip, radiation transmission time of t.sub.0, the total round-trip times between the moment of transmission of the first signal and the moments of reply of the second signal will be t.sub.0 +T.sub.0, t.sub.0 +T.sub.1, . . . t.sub.0 +T.sub.N, for the delays T.sub.0N, T . . . , T.sub.1, respectively. Considering only the transponder delay T.sub.N, at the time t.sub.R, when the second (reply) signal is received at the antenna 56, the frequency 48 of this second signal will be .DELTA.f.sub.N less than the instantaneous frequency 47 of the first signal S1 transmitted by the antenna 56. Thus, if the first and second signals are mixed or "homodyned", this frequency difference .DELTA.f.sub.N will appear in the third signal S3 as a beat frequency. Understandably, other beat frequencies will also result from the other delayed frequency spectra 49 resulting from the time delays T.sub.0, T.sub.1 . . . T.sub.N-1. Thus, in the case of a "chirp" waveform, the difference between the emitted and received waveform will generally be constant.
In mathematical terms, we assume that the phase of a transmitted interrogation signal is .phi.=2.pi.f.tau., where .tau. is the round-trip transmission time delay. For a ramped frequency df/dt or f, we have: 2.pi.f.tau.=d.phi./dt=.omega.. .omega., the beat frequency, is thus determined by .tau. for a given ramped frequency or chirp f. In this case, the signal S3 may be analyzed by determining a frequency content of the S3 signal, for example by applying it to sixteen bandpass filters (of any implementation), each tuned to a different frequency, f.sub.0, f.sub.1 . . . f.sub.E, f.sub.F. The signal processor 102 determines the amplitude and phase of the signals that pass through these respective filters. These amplitudes and phases contain the code or "signature" of the particular signal transformer 40 of the interrogated transponder 20. This signature may be analyzed and decoded in known manner.
The ranges of amplitudes which are expected in the individual components of the second signal S2 associated with the respective pathways or tap delays 0-F may be predicted. The greatest signal amplitudes will be received from pathways having reflectors in their front rows; namely, pathways 0, 1, 4, 5, 8, 9, C and D. The signals received from the pathways having reflectors in their back rows are somewhat attenuated due to reflections and interference by the front row reflectors. If any one of the amplitudes a.sub.i at one of the sixteen frequencies f.sub.i in the third signal S3 falls outside its prescribed or predicted range, as shown in FIG. 5, the decoded identification number for that transponder is rejected.
As indicated above, acoustic transponders are susceptible to so-called "manufacturing" variations, due to intertransponder differences, as well as temperature variations in response due to variations in ambient temperature. This is particularly the case where small differences in tap delays, on the order of one SAW cycle period, are measured to determine the encoded transponder identification number. These manufacturing and/or temperature variations can, in this case, be in the same order of magnitude as the encoded informational signal.
As explained above, the transponder identification number contained in the second (reply) signal is determined, for example, by the presence or absence of delay pads in the respective SAW pathways. These delay pads make a slight adjustment to the propagation time in each pathway, thereby determining the phase of the surface acoustic wave at the instant of its reconversion into electrical energy at the end of its pathway. Accordingly, a fixed code (phase) is imparted to at least two pathways in the SAW device, and the propagation times for these pathways are used as a standard for the propagation times of all other pathways. Likewise, in a reflector-based acoustic device, a reflector may be provided at a predetermined location to produce a reference signal.
The entire process of compensation is illustrated in the flow chart of FIG. 6. As is indicated there, the first step is to calculate the amplitude a.sub.i and phase .phi..sub.i for each audio frequency f.sub.i (block 180). Thereafter, the sixteen amplitudes are compared against their acceptable limits (block 182). These limits may be different for each amplitude. If one or more amplitudes fall outside the acceptable limits, the transponder reading is immediately rejected. If the amplitudes are acceptable, the phase differences .phi..sub.ij are calculated (block 184) and the temperature compensation calculation is performed to determine the best value for .DELTA.T (block 186). Thereafter, the offset compensation calculation is performed (block 188) and the phases for the pathways 2, 3, 6, 7, A, B and E are adjusted. Finally, an attempt is made to place each of the pre-encoded phases into one of the four phase bins (block 190). If all such phases fall within a bin, the transponder identification number is determined; if not, the transponder reading is rejected.
There are a number of other passive remotely readable information bearing devices, such as bar codes, color codes, other types of radio frequency devices, and the like.
Known wireless communications systems include various cellular standards (IS-41, IS-95, IS-136, etc.) as well as so-called PCS standards and data-only standards, including Cellular Packet Data Protocol (CPDP). The Metricom "Ricochet" system provides a frequency hopping 915 MHz spread spectrum wireless local data access system. These communications standards, due to their extensive infrastructure, allow a large number of simultaneous users to communicate over separate communications channels within a relatively small band without substantial mutual interference. Therefore, communications channels may be appropriated for near real time communications needs, such as voice and navigational data.